A widely used type of transistor comprises a body of insulating material which serves as a housing or container for the IC chip which has three leads extending from one of its laterally facing sidewalls. One of these leads has a heat sink on its end which is contained in the insulating housing and the IC chip is bonded to one surface of this heat sink. Conductors, such as fine wires, are bonded to the emitter and base terminal areas of the IC chip and extend to, and are bonded to, the ends of the remaining leads which are also contained in the insulating housing.
The most widely used method of manufacturing transistors of the type described above is to first provide a lead frame of stamped metal having the three leads extending from a carrier member with the heat sink integral with one of the leads. The IC chip is soldered to the heat sink by providing solder between the collector side of the chip and the surface of the heat sink and passing the lead frame through a furnace to melt the solder and bring about the bonding of the chip to the heat sink. Thereafter, the emitter and base areas of the chip are electrically connected to the ends of the remaining terminals or leads, for example, by fine wire bonding, and the heat sink and the end portions of the leads are then encapsulated by molding suitable plastic insulating material onto the lead frame. The carrier strip can then be removed from the ends of the leads which now project from the molded body of plastic.
The manufacture of transistors by the methods described above, or by methods similar to those described above, entails several critical operations which must be carefully controlled. For example, the amount of solder used to bond the IC chip to the surface of the heat sink should be fully adequate to cover the entire lower surface, the collector side, of the IC chip in order to secure a firm bond and to ensure good thermal conductance of heat from the chip. If the amount of solder is inadequate to completely cover the underside of the chip, an inferior bond will result and air pockets between the chip and the heat sink will remain, which will seriously interfere with the transfer of heat to the heat sink. If, on the other hand, an excessive amount of solder is applied, the solder may flow up to the base area of the chip, on the upper side thereof, and thereby short circuit the chip, rendering the finished transistor totally useless. The chip should be precisely placed at a predetermined location on the surface of the heat sink in order to permit the use of automatic bonding equipment in the manufacturing process and slight misalignment or misplacement of the chip can result in bonds being made to the wrong areas of the chip. The molding process in which plastic is molded onto the ends of the leads and the heat sink is extremely critical in that the delicate conductors extending from the chip to the leads can be disturbed during this process and the bonds of these conductors can be broken. It is thus necessary to control all of these operations in order to achieve a good yield from the manufacturing process.
The present invention is directed to the achievement of a lead frame and a manufacturing process which permits improved control of the maufacturing steps and results in the achievement of reductions in the times required and some of the steps and to improved yields in the process.
In accordance with the principles of the invention, premolded housing is provided on the lead frame which encapsulates the inner end portions of the leads and a substantial portion of the heat sink. This housing has a chip-receiving cavity extending therein to the surface of the heat sink, the shape of this cavity conforming precisely to the shape of the chip so that placement of the chip in the cavity locates the chip precisely on the heat sink for a subsequent bonding operation and for the operation of connecting the leads to the chip by fine wires or other means. Solder-receiving binds or bays extend outwardly from the chip-receiving cavity to receive any excess solder above the required for the bonding operation thereby avoiding the possibility of solder flowing to the upper surface of the chip. The chip bonding operation is carried out by means of an induction heating apparatus which produces heat at the chip heat sink interface in a controlled manner, such that the molded housing is not not damaged during the bonding step.